No one with the slightest technical background thought the phone was uncrackable, maybe not directly by the FBI but by pretty much any 3rd party contractor that does even basic flash data recovery. The FBI was looking to set a precedent in order to be able to gain easy access to phones that could be also used by law enforcement, a local sheriff's department is less likely to be able to contract it out for every case and even if it would be a commodity it would be price prohibitive at scale. Overall probably even iPhones with a secure enclave are not "immune" to physical attacks, not as easy as doing simple nand-mirroring but if can decap the main CPU without destroying it you should be able to directly probe the secure enclave logic.

Also while NAND mirroring does work, there is a good chance it wasn't used in that case, the current "theory" is that one of the patched USB DMA exploits were used to unlock the phone, NAND mirroring was too "risky" for the FBI to consider at the time, and it wasn't forensically secure IIRC one of the reasons for the very high price was the non-physical attack approach that the 3rd party provider they've selected had in their toolkit.